SUMMARY The potential of stem cell therapies to restore heart function and promote tissue regeneration in response to injury is evident by the completed and recruiting clinical trials. However, the beneficial effects in these clinical trials using adult CPCs ( are modest. We have demonstrated that neonatal CPCs have superior efficacy in repairing the injured heart compared to aCPCs and recently, revealed that the nCPCs beneficial effect is mediated by a paracrine mechanism through a secretome. Exosomes derived from various type of stem cells/progenitor cells have been shown to mediate stem cell-triggered therapeutic effects on the injured heart through their miRNA cargo. Thus, we hypothesize that HSF1 in nCPCs promotes production of exosomes, functional exosomal miRNAs cargo, and exosome acquisition to recipient cells. The presence of therapeutic miRNAs including miR199a, miR590 and miR146a in nCPC exosomes stimulates cardiomyocyte proliferation and suppresses apoptosis and fibrosis by targeting specific genes leading to the restoration of cardiac function in the injured heart. To test our hypothesis, we proposed three specific aims. Aim 1 will elucidate the functional role of HSF1 in exosome biogenesis and exosomal cargo regulation. We will examine whether HSF1 knockdown in nCPCs will abolish their superior ability in generating functional EXO and EXO cargos, and if HSF1 overexpression in aCPCs will achieve comparable capability to that of nCPCs in EXO therapeutic functionality. Aim 2 will identify the major exosome molecular target in exosome-recipient cardiac cells and determine whether HSF1 is essential for exosome acquisition. We will identify cellular targets of exosomes and if HSF1 is essential for the retention of donor EXOs. We will examine whether Homer1, Clic5, TRAF6 and IRAK1 as EXO major molecular targets in cardiac function recovery. Aim 3 will determine whether the therapeutic miRNAs mediate the effect of exosomes and whether further miRNA enrichment achieves optimal cardiac recovery. We will examine the effects of exosomal miRs-199a, 590, 146a on cardiac recovery.